The present study was carried out to investigate whether the aloe had a radioprotective effect in mice exposed to cobalt-60 gamma radiation or not. The survival ratio of mice for 30 days, hematopoiesis of blood-forming stem cells by spleen colony assay, chromosomal aberration frequency of bone marrow cells and histopathological findings of bone marrow were investigated. The survival ratios of aloe administered groups with concentration of 250, 500, 1,000 and 1,500mg for 3 days before irradiation and control group in cobalt-60 gamma irradiated mice(700rads whole body irradiation, dose rate of 50rads/min.) were 77.4, 79.3, 80.6, 90.0 and 53.1％, respectively. The survival ratios of pre-irradiation aloe administered groups were superior to those of post-irradiation aloe groups and control group. In spleen colony assay, Aloe vera administration before irradiation enhanced the recoveries of numbers of blood-forming stem cells of bone marrow of irradiated mice. There were decreased chromosomal aberrations of bone marrow cells at the first day after irradiation in aloe administered groups compared to that of control group. Histopathological findings in the bone marrow of irradiated mice were hypocellularity due to the depletion of myelocytes, abundant of fat vacuoles and these changes were weakened in aloe administered groups compared to that of control group.